W3C有相关的标准

4.2.1. Nesting '::before' and '::after' pseudo-elements

The selector '::before::before' represents a pseudo-element contained at the start of a pseudo-element contained at the start of an element. For example, the following rules: div { content: 'A' } div::before { content: 'B'; } div::before::before { content: 'C'; } ...would result in the following rendering objects:

 ,-----------------------.
    | ,---------.           |
    | | ,---.   |           |
    | | | C | B | A         |
    | | `---'   |           |
    | `---------'           |
    `-----------------------'

但是Chrome里试了一下好像没效果？什么浏览器支持？还是哪里没写对？

http://codepen.io/tychio/pen/pLDqm